Modern seat belts comprise a length of webbing arranged to pass diagonally across the torso of a vehicle occupant and generally horizontally across the hip region of the vehicle occupant (the so-called lap portion of the belt). This is known as a three point belt system. One end of the belt webbing is firmly attached to a structural part of the vehicle, such as the floor, and the other end is attached to the spool of a retractor which itself is firmly attached to a structural part of the vehicle, usually the side B-pillar. The retractor automatically keeps any slack in the belt wound onto the spool and thus keeps tension in the belt. Between the retractor and the other fixed point, a fastening element such as a metal tongue is fixed to the belt webbing with which it can be fastened into a buckle which itself is attached to a fixed part of the vehicle on the other side of the vehicle seat.
A clock spring in the retractor allows pay out of webbing under the influence of relatively gentle forwardly directed inertia of the vehicle occupant, for example to allow for normal movement of the vehicle occupant such as reaching forward to activate in-car controls, glove compartments or door pockets.
In the event of a crash, the sudden high forward momentum of the vehicle occupant activates a crash sensor which locks the spool against rotation and prevents forward motion of the vehicle occupant to prevent him colliding with the internal fixtures of the vehicle such as the steering wheel, dashboard or windscreen.
However, it has been found in high velocity crashes that the sudden locking of the seat belt can itself cause injury to the vehicle occupant due to the sudden impact of the torso with the belt webbing.
In recent years it has been proposed to introduce a load limiting effect into the seat belt system so as to allow a limited and controlled forward motion of the vehicle occupant after the retractor has locked. This decreases the forces exerted by the belt on the vehicle occupant's torso.